Ocean Acidification Explained

The Chemistry of Acidification

When CO2 dissolves in seawater, it reacts with water to form carbonic acid (H2CO3). This weak acid quickly dissociates, releasing hydrogen ions (H+) and bicarbonate ions (HCO3-). The increased hydrogen ion concentration directly lowers pH. In normal seawater chemistry, bicarbonate can further dissociate to carbonate ions (CO3 2-), but as pH drops, the equilibrium shifts toward bicarbonate at the expense of carbonate. This reduction in carbonate ion concentration is critical because many marine organisms need carbonate ions to build their shells and skeletons.

The ocean's carbonate chemistry acts as a buffer, resisting pH changes by converting dissolved CO2 into bicarbonate and carbonate. This buffer capacity is what allows the ocean to absorb so much CO2 without even larger pH changes. However, as the buffer is consumed by absorbing more CO2, its effectiveness decreases. The ocean's ability to absorb additional CO2 is declining with each increment of concentration increase.

The term "acidification" refers to the direction of pH change (toward more acidic), not the absolute state. Seawater remains alkaline (pH above 7) and is not expected to become truly acidic under any realistic emissions scenario. However, even small pH changes have large biological effects because organisms have evolved within a narrow pH range over millions of years.

Biological Impacts

Coral reefs are among the most vulnerable ecosystems. Reef-building corals secrete aragonite, a form of calcium carbonate that becomes harder to produce as carbonate saturation decreases. Under projected conditions, many tropical waters will become undersaturated with respect to aragonite by mid-century, making coral skeleton formation energetically costly. Combined with warming-induced bleaching, acidification threatens the existence of coral reef ecosystems that support 25 percent of all marine species.

Pteropods (small swimming snails) are particularly vulnerable because their thin aragonite shells dissolve in undersaturated water. These organisms are a key food source for salmon, herring, and other commercially important fish in polar and subpolar waters. Laboratory experiments show visible shell dissolution at CO2 levels projected for 2050, and field studies have already documented shell thinning in living pteropods from the Southern Ocean.

Shellfish including oysters, mussels, and clams face reduced calcification rates. Oyster hatcheries on the US Pacific coast experienced massive larval die-offs in the 2000s when upwelled acidified water entered their facilities, providing an early warning of economic impacts. The shellfish industry has adapted by monitoring water chemistry and buffering hatchery water, but wild populations cannot employ such strategies.

Some organisms may benefit from higher CO2. Seagrasses and some algae can increase photosynthesis under elevated CO2 conditions. However, the overall ecosystem effects are expected to be negative, with loss of structural organisms like corals and reduced food web support from calcifying plankton.

Geographic Patterns

Acidification is most severe in cold, high-latitude waters because cold water absorbs more CO2 and has naturally lower carbonate saturation. The Arctic Ocean is projected to become undersaturated with respect to aragonite across large areas within decades. Upwelling zones along continental margins also experience enhanced acidification as deep, CO2-rich water reaches the surface.

Interaction with Warming

Acidification and warming interact to compound stress on marine organisms. Coral bleaching from heat stress reduces the organism's energy reserves, making it less able to cope with the additional metabolic cost of calcifying in more acidic water. Many marine species face simultaneous pressure from warming, acidification, and deoxygenation, creating multi-stressor environments with limited analog in Earth's recent past.